Iestyn wrote:What, in a situation, encourages you to speak Welsh? What things do people say or do, or what kind of atmosphere helps? Could you give pointers to someone who wants to encourage learners to use Welsh with them?
When people address me in Welsh (knowing I'm a learner or not). Even if we switch to English eventually it is a small victory.
People who know I'm a learner and are fishing for me to speak Welsh or just patiently give me some time to get the words out.
Rephrasing words when my brow furls!

Iestyn wrote: And what switches you off?
People who swap to English as soon as it is clear that I'm not a fluent speaker.
I can often keep up with a conversation even when I am too tired to speak Welsh myself, or when the topic is too complicated (e.g. politics and government in my case). I'm happy for poeple to carry on in Welsh and I'll mix English and Welsh as I need to. It is annoying when people then still go over to English, eventhough I've just said "Na, na mae'n iawn. Cari ymlaen. Dw i'n deall."
The best thing is to get to know people in Welsh! It means your brain is somehow expecting Welsh and it makes it much easier for both parties to stick to Welsh. Some Welsh speakers I know have said that it's very hard for them to switch language becasue they see my face and English comes out. Luckily most of them are now use to speaking Welsh when I am around (they know I can keep up) and that makes it easier for me to use my still limited Welsh.